1) Field of the Invention
The present invention relates generally to floating vessels, both traditional “ship-shaped” vessels and semi-submersible vessels. The invention relates more particularly to a method of installing a tension leg platform and connecting it to mooring tendons/tethers and connecting the tendons to foundations, such as driven or drilled piles, suction piles or suction gravity caissons, which are anchored in the seabed.
2) Description of the Prior Art
In the offshore oil and gas industry, floating vessels such as tension leg platforms (TLPs) for drilling and/or production are common. A TLP is a type of floating platform that is used for drilling and production in relatively deep water. The TLP is moored using vertical tendons (also referred to as tethers) connected to foundations anchored in the seabed. The tendons are tensioned by the buoyancy force of the TLP hull, which is submerged or partially submerged.
Depending on its configuration, the stability of a TLP with or without an integrated deck may be inadequate during installation. When a TLP is ballasted between the initial free floating draft (e.g. the wet-tow draft or float-off draft) and the lock-off draft (the draft at which securing the TLP to the tendons is initiated), there is a range of drafts at which the TLP stability is critical—the TLP may be unstable or marginally stable prior to being locked off to the tendons. There are a number of ways to make the TLP stable. For example, a combination of wider column spacing and/or larger columns may be used to increase stability. Alternatively, the topsides deck may be installed offshore after the hull is connected to the tendons. Offshore installation of the deck is an expensive, high-risk operation and requires good weather.
Because of the stability concerns of a TLP when transiting the installation drafts before being locked off, prior art installation techniques have often relied on using costly specialized installation equipment such temporary buoyancy modules to keep the hull from capsizing before it can be secured to its mooring tendons and subsequently de-ballasted.
Another method to maintain stability is the use of an upward hook load to the TLP by a larger installation support vessel. A hook load has the advantage of being able to quickly tension the tendons after lock-off without waiting for the slow de-ballasting process. However, only a a very limited number of vessels exist worldwide which are capable of providing the required hook load for a TLP of ordinary size.
However, U.S. Pat. No. 5,551,802 describes a method which overcomes the need for special installation equipment and allows the TLP to be installed with just a conventional deep water drilling vessel and assist tugs. After the TLP is towed over the preinstalled mooring tendons, it is held in position by deep water drilling vessel and tugs. As the hull ballasts, it is held with downward tension near each connector sleeve (sometimes known as a slip nut or slips assembly) by tensioning lines, attached to the tips of the tendons, passing through the corresponding connection sleeves and passing through ratcheting cleats or grippers mounted directly above the connection sleeves. The tensioning lines are tensioned by constant tension devices. The grippers serve to check any upward movement. For the unstable hull to capsize, one side must rotate up, which is not possible when downward tension is applied at the various connection points.
While this latter-described prior art method has many advantages over its predecessors, because the grippers are mounted on the hull below the waterline, the method suffers from risk of gripper slippage, difficult gripper installation, operation, maintenance and removal. Rigging the tensioning lines can be problematic. Further, because grippers do not allow selective paying out of line, high transient loads can occur. It is desirable to be able to haul in and pay out line during installation to maintain the tensioning lines within a window of safe operating tensions.
Further, it is desirable to minimize the time required for installation by reducing the amount of ballasting and deballasting (i.e. ballast manipulation) required to install the TLP. By reducing the ballast and de-ballast times, the time the TLP is at risk to weather and instability is also reduced.
3) Identification of Objects of the Invention
A primary object of the invention is to provide a method of TLP installation, which provides stability to TLP during transit through the various installation drafts without the need for hook loads or temporary buoyancy modules.
Another primary object of the invention is to provide a motion-arresting capability that reduces the TLP heaving motions at the TLP drafts close to the lock-off draft, and enables a safe and simultaneous lock-off of the tendons to the hull.
Another primary object of the invention is to provide a TLP installation system which aids in TLP station keeping during the installation process.
Another primary object of the invention is to provide a system for rapidly submerging the TLP hull without ballasting or with minimal ballasting and/or ballasting manipulation to minimize the time during which the TLP is made to transit the TLP installation drafts. By eliminating or reducing ballasting, the required tendon pre-tension can be rapidly achieved after tendon lock-off without the need for a lengthy de-ballasting process.
Another object of the invention is to provide a method for the installation of a TLP hull with an integrated deck. When the deck is integrated with the hull onshore, pre-commissioning is possible which saves offshore commissioning time and reduces the risks as well as costs associated with marine installation. The invention eliminates the need to use a crane vessel, derrick barge or other lifting mechanism for offshore deck installation and can therefore reduce the installation cost.
Another object of the invention is to provide a method for installation of a TLP with an integrated deck in potentially higher seastates than is normally allowable for offshore lift installation of the deck, for installation with the use of temporary buoyancy modules, or for installation using an upward hook load to the TLP by a larger installation support vessel.
Another object of the invention is to provide a method of TLP installation equally suitable for a TLP hull with or without a pre-installed deck, or for installation of a semi-submersible or any floating platform wherein the tendons are replaced by vertically-tensioned chains or wire ropes, synthetic lines or other equivalent.
Another object of the invention is to provide a TLP installation system which minimizes the time during which the TLP can have a resonant frequency with external exciting system (e.g. wave frequencies of the surrounding water).
Another object of the invention is to provide a TLP installation system in which the major components can be easily removed after TLP and riser installation.
Another object of the invention is to provide a TLP installation system having minimal underwater components.
Another object of the invention is to provide a TLP installation method which can be used to aid in the installation of the tethers, thus eliminating the need for tendon support buoys.